It remains a challenging issue to localize blocked RFID objects. Existing solutions rely on non-line-of-sight (NLOS) signals reflected from environments, which are not reliable and cannot be accurately measured. To eliminate such limitations, this paper develops a new approach (called MetaSight) that localizes blocked objects by modulating NLOS signals via metasurfaces. More specifically, a programmable metasurface is designed such that each reflecting element can dynamically change frequencies and phases of reflected signals. With proper setting of frequencies and phases within a certain range of reflecting elements (i.e., the reflecting window) by a reflection beamforming algorithm, a unique NLOS signal path (called metasurface path or MS path) is established between an object and its reader. By shifting the reflecting window in time domain, multiple MS paths and the corresponding channel coefficients can be obtained sequentially. To make the sequential process fast, both the preamble and the payload of a frame are utilized for channel estimation. Based on the estimated channel coefficients of multiple MS paths, the 3D direction of the object viewed from the metasurface is derived through a 3D direction estimation algorithm. By consolidating the 3D directions from two separate metasurfaces, the object's 3D location is finally determined. MetaSight is implemented as a prototype system. Extensive experiments show that MetaSight can localize a blocked object with an average accuracy of 15 cm.

Localizing RFID-tagged objects by a mobile robot plays an important role in many Internet of Things (IoT) applications. Existing RFID localization systems are infeasible, since they either demand bulky RFID infrastructures or cannot achieve sufficient localization accuracy. In this article, a portable localization (POLO) system is developed for a mobile robot to locate RFID-tagged objects. POLO consists of an RFID reader, a tag array, and a lightweight receiver. The reader is used for interrogating the RFID tag on an object. The tag array is designed to reflect the RFID signal from an object into multipath signals. The receiver captures such signals and estimates their multipath channel coefficients by a tag-array-assisted channel estimation (TCE) mechanism. Such channel coefficients are further exploited to determine the object’s direction by a spatial smoothing direction estimation (SSDE) algorithm. To resist the impact of multipath reflections from surroundings, more spatial information is exploited by placing the tag elements densely and collecting the channel coefficients during the robot’s movement. Based on the object’s direction, POLO guides the robot to approach the object. When the object is in proximity, its 3-D location is finally determined by a near-range positioning (NRP) algorithm. Moreover, POLO is designed to be compatible with commercial RFID systems. POLO is prototyped and evaluated via extensive experiments. Results show that the average angular error is within 1 degree when the object is in the far range (2–6 m), and the average location error is within 6 cm while the object is in the near range (~1 m).

To meet the tremendous demand of Internet of Things (IoT) applications, the 3rd generation partnership project (3GPP) has specified the narrowband IoT (NB-IoT) standard. However, collisions in the random access (RA) channel of NB-IoT can be severe due to the mismatch between frequent random access attempts by a huge number of devices and the limited radio resources. In this paper, a new random access mechanism called time-alignment-value based random access (TARA) is designed to improve the efficiency of random access of NB-IoT. The key mechanism of TARA is to conduct quick retries upon failure by exploiting the difference of time-alignment (TA) values in collided preambles. To analyze TARA rigorously, a theoretical model of TARA is derived, and its validity is verified via simulations. Further simulations are carried out to evaluate TARA with respect to different system parameters. Comparisons with existing schemes are also conducted. Both analytical and simulation results show that, under various system parameters, TARA achieves 30 percent higher success probability of random access, 75 percent higher throughput, and 40 percent lower access delay than the original slotted Aloha mechanism of NB-IoT. It also highly outperforms other random access schemes.

In many Internet-of-Things (IoT) applications, various RFID-tagged objects need to be localized by mobile robots. Existing RFID localization systems are infeasible, since they either demand bulky RFID infrastructures or cannot achieve sufficient localization accuracy. In this paper, a portable localization (POLO) system is developed for a mobile robot to locate RFID-tagged objects. Besides a single RFID reader on board, POLO is distinguished with a tag array and a lightweight receiver. The tag array is designed to reflect the RFID signal from an object into multi-path signals. The receiver captures such signals and estimates their multi-path channel coefficients by a tag-array-assisted channel estimation (TCE) mechanism. Such channel coefficients are further exploited to determine the object's direction by a spatial smoothing direction estimation (SSDE) algorithm. Based on the object's direction, POLO guides the robot to approach the object. When the object is in proximity, its 2D location is finally determined by a near-range positioning (NRP) algorithm. POLO is prototyped and evaluated via extensive experiments. Results show that the average angular error is within 1.6 degrees when the object is in the far-range (2~6 m), and the average location error is within 5 cm while the object is in the near-range (~1 m).

Existing physical layer security schemes are inapplicable to legacy devices operating on long coherence-time channels, since they demand changes in the physical layer. To this end, a new physical-layer security scheme is developed to secure legacy clients. In this scheme, secret keys are generated by a client and transmitted to the access point (AP). To protect these keys, a separated device called secrecy protector (SP) transmits jamming signals to prevent eavesdroppers from overhearing the keys. The SP is equipped with multiple antennas, each of which transmits an independent jamming stream with a pseudo-preamble. Since the SP can share jamming signals with the AP secretly, the AP can use a certain analog network coding scheme to remove jamming signals and decode the keys. In contrast, eavesdroppers have no knowledge of jamming signals to decode the keys. However, the long coherence-time channels are vulnerable to eavesdroppers, as they can guess the channel coefficients in a brute force way and then remove the jamming signals. Thus, frequency diversity is exploited to enhance the security of the system. Moreover, the design of jamming mechanisms resists collusion among eavesdroppers. Therefore, secret keys can be secretly shared between the AP and a client. The developed scheme is implemented on a software-defined radio platform. Performance results demonstrate that it can effectively deliver secure communications without any changes to the physical layer of legacy clients.

IEEE 802.11ax has adopted orthogonal frequency division multiple access (OFDMA) to support multi-user (MU) transmissions. There exist two uplink MU OFDMA access methods in IEEE 802.11ax. The first one is uplink OFDMA random access (UORA) in which stations randomly select resource units (RUs) to send physical protocol data units (PPDUs), so its access efficiency is low. The second one is uplink OFDMA nonrandom access (UONRA) in which the access point (AP) schedules MU transmissions based on buffer status reports (BSR) from stations. However, stations usually rely on UORA to send BSRs, so the low efficiency of UORA can be a bottleneck of UONRA. Thus, UORA needs to be renovated to improve the performance of itself and UONRA. To this end, a multi-dimensional busy-tone arbitration (MBTA) mechanism is developed in this paper to reduce collisions among stations contending the same RU. Since there lacks an algorithm in IEEE 802.11ax to support coexistence of UORA and UONRA, a dynamic access-method selection (DAMS) algorithm is designed for the AP and stations to choose an optimal access method. Both MBTA and DAMS are analyzed rigorously and are further validated via simulations. The analytical and simulation results show that: 1) The MBTA dramatically improves the access efficiency of UORA; 2) DAMS always achieves a higher throughput than both UORA and UONRA.

RFID-based passive positioning is essential for many Internet-of-Things applications. However, existing RFID-based passive positioning systems either have low precision or require multiple antenna arrays, which are bulky and expensive. In this paper, a 3D passive positioning scheme is developed based on RFID tag arrays. One of the tags can harvest the energy of the surrounding electromagnetic waves to power the embedded orientation sensors (an accelerometer and a magnetometer) and then send the orientation information of an object to an RFID reader. When an object is equipped with such RFID tag arrays, its direction can be estimated by exploring the phases of the signals reflected by the tags and the orientation of the object. Furthermore, based on the triangulation principle, the 3D location of an object can be determined by using multiple RFID tags on the object and two antennas at the RFID reader. Thus, compared with existing schemes, the required number of RFID antennas for 3D passive positioning is greatly reduced. The proposed scheme is implemented based on a COTS RFID platform. The experimental results show that the 90-percentile accuracy of the proposed system is within 9 centimeters.

To meet the tremendous demand of Internet of Things (IoT) applications, the 3rd Generation Partnership Project (3GPP) has specified the Narrowband IoT (NB-IoT) standard. However, collisions in the random access (RA) channel of NB-IoT can be severe due to mismatch between frequent random access attempts by a huge number of devices and the extremely limited radio resources. In this paper, a new random access mechanism called time-alignment-value based random access (TARA) is designed to increase the efficiency of the random access channel. The key idea of TARA is to conduct quick retries by an RA-failure device based on the time-alignment (TA) values of colliding UEs in a random access response (RAR) message. Simulation results show that TARA performs much better than the original slotted Aloha mechanism, i.e., higher success probability, higher throughput, and lower access delay. Comparison with other existing schemes further demonstrates high performance and advantages of TARA.